I agree, I think you are getting ahead of yourself. If she was taking dust baths like a one year old just a week ago, I would doubt arthritis would so suddenly cause her not to be able to roll. Perhaps she has an injury of some sort, the bath is too small for her to feel comfortable rolling wildly, she just doesn't feel like it...any number of reasons. Take her to a competent vet that can give her a thorough exam if you feel she needs it, but if everything else is normal for her, eliminating waste, eating, drinking, weight maintenance, energy level, then I would just let her do her thing and help her out if she needs it. Try a different dust bath or let her roll in it on the floor...